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Abstract

The ability of 8 picosecond pulse lasers for three dimensional direct-writing in the bulk of transparent dielectrics is assessed through a comparative study with a femtosecond laser delivering 600 fs pulses. The comparison addresses two main applications: the fabrication of birefringent optical elements and two-step machining by laser exposure and post-processing by chemical etching. Formation of self-organized nano-gratings in glass by ps-pulses is demonstrated. Differential etching between ps-laser exposed regions and unexposed silica is observed. Despite attaining values of retardance (>100 nm) and etching rate (2 μm/min) similar to fs pulses, ps pulses are found unsuitable for bulk machining in silica glass primarily due to the build-up of a stress field causing scattering, cracks and non-homogeneous etching. Additionally, we show that the so-called “quill-effect”, that is the dependence of the laser damage from the direction of writing, occurs also for ps-pulse laser machining. Finally, an opposite dependence of the retardance from the intra-pulse distance is observed for fs- and ps-laser direct writing.

Figures (9)

(a) Optical image of a set of laser tracks radiating out from a virtual circle (∅ = 100 μm). The angle between lines is 15 deg. The Laser polarization is parallel to the writing direction. λ = 1064 nm, NA = 0.65, τ = 8 ps, Ep = 2 μJ, f = 200 kHz, v = 200 μm/s. (b) and (c) Quantitative birefringence measurements of the laser tracks in (a). (b) Pseudo colors represent the angle of the slow axis of birefringence as indicated by the legend in the top right corner. (c) Pseudo colors represent the strength of the retardance. (d) Retardance vs. position along the dashed line in (c). The retardance measured between two laser tracks is indicative of stress-induced birefringence.

Comparison of the length of the micro-channel obtained in silica processed by fs- and ps-laser after 80 min in 2.5% HF bath. Each data point in the plot is the average of the etched length of channels exposed at the same net fluence and the error bars give the corresponding standard deviation. Hence, a small error bar indicates that the etched length has little dependence on laser frequency and scanning speed.